A major life threatening hazard associated with intravenous infusion systems is a blockage which prevents the infusion of the life saving media. Many monitoring systems exist to detect this hazard. Typical systems available work by mechanical linkages and changes in optical transmisivity. The system described herein uses the concept of total internal reflection by devices to be described in more detail. A major use of the phenomena of internal reflection is for light intensity modulation. However, it has also been used for liquid level detection, for pure fluid annunciator, and for fine measurement of pressure. The physics of total internal reflection is well described in the Patent of Whitsel (U.S. Pat. No. 3,517,639). Given: an isosceles triangular prism with two equilateral planes and its adjacent hypotenuse plane; for the sake of discussion, assume the prism is a right triangular glass prism. A light source is positioned so as to be incident normally on one of the perpendicular equilateral plane faces of the right angle prism. This means that the transmission of light rays will be incident on hypotenuse at an angle of substantially 45.degree.. Accordingly, Snell's Law may be written as nsin .theta. is =n' sin .theta.' where n and n' are the indices of refraction of the prism material and the media present against the hypotenuse plane, and .theta. and .theta.' are the angles of incidence and refraction respectively. To find the critical angle which the refracted rays have an angle of 90.degree., that is .theta.' is =90.degree., it would follow the equation would be sin .theta..sub.C =n'/n, where .theta..sub.C is the critical angle. If the glass prism has an index of refraction of 1.5, and air is in contact with the hypotenuse plane, air having an index refraction of 1.00, the critical angle, .theta..sub.C for air is 42.degree.. Light incident at 45.degree. will be totally reflected in the prism to the other equilateral plane surface. If a plastic material makes contact with the hypotenuse plane, the critical angle changes. For example, if the index of refraction of the plastic is 1.33, the critical angle .theta..sub.C becomes 62.degree.. Since the incident ray is set at 45.degree., which is less than the required incident angle of 62.degree., all of the incoming light will be refracted through the hypotenuse plane into the plastic and no light rays will be reflected through the other equilateral planes surface.